Field of the Invention
The invention relates to a method for structuring a photoresist layer.
In semiconductor technology, photolithographic methods play a key role in the production of integrated circuits on a semiconductor substrate. Typically, photoresist layers are applied to the surface of the substrate to be structured and are then structured by exposure to radiation from a suitable wavelength range. The exposure for structuring is effected by a lithography mask that predetermines the structure that is to be transferred to the substrate. The exposed parts of the photoresist layer are chemically modified by the exposure and their polarity is thus changed. For this reason, the exposed and the unexposed parts of the photoresist have different solubilities in a corresponding developer. This is used in the subsequent development step for selectively removing the exposed or unexposed parts. Those parts of the photoresist layer that remain on the substrate serve in the following structuring step as a mask which protects the substrate layer underneath from removal of material or modification of material. Such a structuring step may be, for example, plasma etching, wet chemical etching or ion implantation.
Both in the case of the one-layer resists developable by a wet method and in the case of the two-layer resist systems developable completely or partly by a dry method, chemical amplification resists (CAR) have proven particularly useful as photoresists. Chemical amplification resists are characterized in that they contain a photo acid generator, i.e. a photosensitive compound, which generates a protic acid on exposure to light. The protic acid then initiates acid-catalyzed reactions in the base polymer of the resist, if necessary with thermal treatment of the resist. As a result of the presence of the photo acid generator, the sensitivity of the photoresist is substantially increased compared with a conventional photoresist. An overview of this topic is given by H. Ito in Solid State Technology, July 1996, page 164 et seq.
In the case of the positive resists, the different solubilities of the exposed and of the unexposed photoresist is achieved by the principle of acid-catalyzed cleavage. A polar carboxyl group is formed thereby from a nonpolar chemical group of the layer-forming polymer, for example a tert-butyl carboxylate group, in the presence of a photolytically produced acid, if necessary in a heating step.
Further examples of nonpolar “blocked” groups which can be converted into corresponding polar groups by acid-catalyzed reactions are the tert-butoxycarbonyloxy (t-BOC) or acetal groups. Through the conversion of the nonpolar group into the corresponding polar group, the resist undergoes a change in polarity in the previously exposed parts, with the result that it becomes soluble in the polar, aqueous alkaline developer. Consequently, the exposed parts of the photoresist can be selectively removed by the developer. The resist residues in the unexposed, nonpolar parts thus geometrically define a resist profile or a resist structure on the substrate, which serves in subsequent process steps as a mask for surface structuring.
In contrast, a reduction in the solubility of the photoresist in the exposed parts is caused by the exposure in negative resists. In order to achieve this, negative-working photoresists have, as a rule, cross-linkable groups that can undergo cross-linking reactions under the influence of irradiation. As a result of the cross-linking, the solubility of the exposed parts of the photoresist in a corresponding developer is reduced. The cross-linkable groups can be either directly bonded to the base polymer or be present as a separate cross-linking component in the photoresist. In chemically amplified, negative-working photoresists, groups which are cross-linkable under acid catalysis and are activated by the photolytically liberated acid are used.
Owing to the constantly increasing integration density in semiconductor technology, the accuracy with which the resist profile can be produced after development on a surface to be structured is of decisive importance. The resist profile is first physically uniquely predefined by the light distribution during the exposure. Second, it is chemically transferred to the resist layer by the distribution of the components produced photochemically by the exposure.
Owing to the physicochemical properties of the resist materials, completely unfalsified transfer of the pattern predetermined by the lithography mask to the resist is, however, not definitively possible. Here, in particular interference effects and light scattering in the photoresist play a major role. However, the steps following the exposure, such as, for example, the development, additionally have a considerable influence on the quality of the resist profiles, which is substantially determined by the profile sidewalls. In order to achieve surface structuring that is as precise as possible in the subsequent process steps, it would be ideal if it were possible to obtain virtually perpendicular, smooth profile sidewalls in the resist profile after development of the photoresist.
In particular, the light intensity profile that results in the photoresist during the exposure has an adverse effect on the steepness of the profile sidewalls that is to be achieved. This characteristic intensity profile, which is also referred to as “aerial” image, is due to the light scattering and light absorption occurring in the resist during exposure. Since the photoresist absorbs a certain proportion of the incident radiation, the observed radiation intensity decreases with increasing layer thickness in the photoresist. Consequently, those parts of the photoresist layer which are close to the surface are more strongly exposed. In the case of a positive resist, the parts that are close to the surface are thus more highly soluble than the parts away from the surface. The different solubilities within an exposed part of the resist often leads to a flattening and to poor definition of the profile sidewalls in the case of positive resists. The light intensity profile in the photoresist thus describes the distribution of a photochemically modified species, for example the distribution of the photolytically produced acid in the photoresist in the case of a positive resist.
In the case of negative resists, the decrease in the radiation intensity with increasing layer thickness in the photoresist leads to greater cross-linking in the parts which are close to the surface and which thus have a lower solubility than the parts away from the surface. In the subsequent development of the exposed photoresist, those parts of the photoresist layer which are away from the surface are thus dissolved to a greater extent than the parts on top which are close to the surface, with the result that the quality of the profile sidewalls and hence the resolution also deteriorate.
The quality and the steepness of the resist profiles are of decisive importance for the transfer of the structure from the photomask to the layer underneath which is to be structured. One known approach for improving the quality of resist profiles in positive resists is described in Published, European Patent Application EP 962 825 A. There, an improved steepness of the resist sidewalls is achieved by adding to the photoresist two photochemically active additives that are activated by radiation in different wavelength ranges in each case. First, the photoresist contains a photo acid generator which, as already described above, liberates an acid on exposure to light of a defined wavelength range, which acid then catalyzes the reaction of the convertible nonpolar groups of the layer-forming polymer of the photoresist to give carboxyl groups and hence causes the photoresist to be soluble in the polar developer. Second, the photoresist contains, as the second photochemical additive, a cross-linking reagent which results in a reduction of the solubility of the photoresist. The cross-linking reagent is likewise activated by radiation, the radiation used for this purpose differing from the radiation used for activating the photo acid generator.
According to Published, European Patent Application EP 962 825 A, the photo acid generator is activated in the ranges determined by the mask layout, in a first exposure step for structuring. In a subsequent, second floodlight exposure step, the total photoresist layer is exposed without the use of a photomask, and the cross-linking reagent is thus photochemically activated over the total surface of the photoresist layer. As a result of the chemical cross-linking of the photoresist which is thus initiated, its solubility is reduced. Since those parts of the photoresist that are close to the surface are more strongly exposed, they are more strongly cross-linked and hence more insoluble than the parts away from the surface. This change in the solubility is opposite to the change in solubility that is achieved in the first exposure step. While the exposed parts close to the surface have a higher solubility than the parts away from the surface as a result of the first exposure step, precisely the opposite gradient is produced by the second exposure step. Owing to the selective solubility modification in the photoresist, increased developer selectivity in the aqueous developer is achieved, resulting in steeper resist profile sidewalls.
However, this approach has a decisive disadvantage: the cross-linking reaction leads to the formation of a three-dimensional network polymer, particularly in those parts of the photoresist which are close to the surface. The network polymer has altered development behavior compared with the original, linear layer-forming polymer, which leads to “rough”, i.e. inexactly defined, e.g. frayed profile sidewalls. This roughness complicates the subsequent process steps, such as, for example, the substrate etching.